A mobile robot senses objects in its environment in order to determine its own location, map the environment for later localization and navigation, and, in non-static environments, to avoid obstacles.
A number of obstacle detection systems are currently in use, including complex computer vision systems, scanning laser range finders, and arrays of discrete ultrasonic transducers. Computer vision and scanning laser range finders tend to be prohibitively expensive in many applications. Mobile robots therefore often use ultrasonic sonar devices for obstacle avoidance.
The word “sonar” is an acronym for “SOund Navigation and Ranging”. A sonar scanner typically includes a transmitter, a transducer, a receiver, and a signal processor. An electrical impulse (or “pulse”), usually a waveform, from the transmitter is converted into a sound wave by the transducer and propagated in a given direction. When the sound wave strikes an object, it rebounds, producing an “echo”. This echo strikes the transducer. The transducer converts the echo into an electrical signal, which is amplified by the receiver and fed into a detector and stored for processing.
Since the speed of sound in any given environment is known, the time lapse between the transmitted signal and the received echo can be measured and the distance to an object that produced the echo determined. This time lapse is called the “time of flight”.
One configuration for producing the transmitted signal and receiving the echo is a sonar ring. A sonar ring includes a number of transducers positioned around a robot to allow simultaneous sensing in more than one direction. A sonar ring may include any number of transducers and may or may not provide detection in a 360° field, depending on the number, placement and echo field of the transducers. For example, where each transducer's echo field covers 15° of radial space, a robot would need 24 evenly-spaced and directed transducers in order to have full 360° coverage.
This high number of sonar components has a number of drawbacks, including cost. Not only do sonar ring systems eliminate many of the cost benefits of using a sonar-based system, sonar rings often produce data of questionable quality due to the increased amount of acoustic energy being transmitted into the environment from multiple transducers. This acoustic energy may reflect off of multiple objects, resulting in multipath reflections. Multipath reflections are echoes that have not traveled in a direct path from the object to the receiver. For example, an echo that reflects off of one or more walls will have a longer time of flight and appear as an object farther away from the sonar scanner than it is in fact.
Thus, the transducer is subject to receiving sound waves from several sources, not just the intended echo. Distinguishing between the echo, spurious signals and/or extraneous noise is one challenge faced in designing a sonar scanner for a mobile robot or other application. In addition to multipath reflections, examples of extraneous environmental noise include acoustic steady state and periodic noise, acoustic impulse noise, electrical impulse, and ultrasonic noise from intentional sources, such as other robots, ultrasonic door openers, etc.
In a sonar scanner, the transducer emits a signal (or pulse) having a finite duration, temit. Conventionally, during the process of emitting the pulse, the receiver is disabled by a system controller as it waits for the transducer to cease operation in order to begin listening for the echo. The time that the receiver is disabled is known as the “blanking time” tblank. The receiver is disabled in order to be sure that it does not become saturated as it detects the transducer emitting the sound wave. A byproduct of this delay, i.e., the receiver blanking time, is that it prevents the sonar scanner from detecting real objects within a certain distance.
Typically, the duration of the pulse signal is reduced in order to reduce the minimum measurable distance of the sonar scanner. Due to its mechanical nature, however, the transducer continues to vibrate even after the transmitter signal has ceased. This continued vibration is typically referred to as the “ring-down”. This additional latency in the system extends temit, so that the total time of the signal can be characterized as the duration of the pulse (tpulse) plus the duration of the ring-down (tring-down). Thus, the ring-down makes it difficult for a sonar scanner to detect objects within a certain distance. In the prior art, the minimum detection distance is about one foot.
In addition, mobile robot sonar scanners also face challenges from extraneous noise in the environment. For example, if the environment contains a continuous source of acoustic or electrical noise, such as a vacuum cleaner or air conditioning unit, the sonar scanner may be susceptible to this noise. The robot itself may be a source of continuous noise, which could also affect the ability of the sonar scanner to detect echoes.